1. Field of the Invention
The invention concerns apparatus for the measurement of the velocity of mechanical low frequency vibrations or oscillations, especially of mechanical vibrations of the earth which can in certain circumstances cause damage to objects or articles in the environment, depending on their condition.
2. Description of the Prior Art
So-called geophones or seismophones have been used to measure the magnitude of the vibrations. For buildings, particularly those low frequency vibrations are harmful which have a frequency of the same order of magnitude as the natural frequency of these buildings. The specifications relating to the permissible vibration magnitudes differ according to the respective stipulations of individual countries. In Hungary the highest permissible velocity of vibration sets the standard.
Geophones or seismophones sensitive to the velocity of low frequency vibrations are manufactured in virtually every industrially developed country. Yet the price of these devices is very high. The high cost can be explained by the use of special materials and by the indispensable precision work required. Further, it is a drawback of these devices that they can easily be damaged. A low natural frequency can only be achieved by means of a relatively large weight or mass and a small spring constant.
Geophones or seismophones of special construction have a natural frequency of 2 to 3 Hz. A predetermined natural frequency and a given spring constant are associated with a definite mass. In order to enable the vibrating mass to be held to a low frequency, the known devices are designed with a large spring diameter (up to 12 cm) whereby, however, the dimensions of the apparatus are unfavourably influenced.